Production of high surface area mesoporous activated carbons from waste biomass using hydrogen peroxide-mediated hydrothermal treatment for adsorption applications

Chemical Engineering Journal 273 (2015) 622–629

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Chemical Engineering Journal

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Production of high surface area mesoporous activated carbons fromwaste biomass using hydrogen peroxide-mediated hydrothermaltreatment for adsorption applications

http://dx.doi.org/10.1016/j.cej.2015.03.1111385-8947/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author.

Akshay Jain a, Rajasekhar Balasubramanian b, M.P. Srinivasan a,⇑a Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singaporeb Department of Civil and Environmental Engineering, National University of Singapore, Singapore 117585, Singapore

h i g h l i g h t s

� High mesopore area obtained byimproved chemical activation withZnCl2.� Hydrothermal treatment of biomass

with H2O2 enhanced the chemicalactivation.� Up to 100% increase in mesopore area

is achieved by employing H2O2 pre-treatment.� OFG in biomass/precursor and

mesopore area in the carbon arestrongly correlated.� High dye uptake capacity of 714 mg/g

is achieved.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 January 2015Received in revised form 23 March 2015Accepted 23 March 2015Available online 28 March 2015

Keywords:Hydrothermal carbonizationMesoporous activated carbonChemical activationOxidizing agentHydrogen peroxideDye adsorption

a b s t r a c t

High-surface area mesoporous activated carbons were prepared from biomass by incorporating H2O2 asan oxidizing agent during hydrothermal pre-treatment of the raw material (coconut shell). Use of H2O2

led to enhanced formation of OFGs in the hydrochar precursor and resulted in activated carbons possess-ing high mesopore and BET surface areas. A strong interdependence is observed between the extent offormation of oxygenated functional groups (OFGs) in the hydrochars obtained from the hydrothermalpre-treatment and mesopore area in corresponding carbons. The mesoporous carbons showed highadsorption capacity for Rhodamine B up to 714 mg/g.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Porous carbon-based materials serve as the material of choice inmany applications such as gas separation [1–3], water and air pur-ification [4–6], catalysis [7,8], chromatography [9], energy storage[10–12], electrode materials [7,13–18], hydrogen storage [19,20].

In particular, mesoporous carbon materials are critical for applica-tions which involve large molecules, such as substrates forimmobilizing biomolecules, electrodes for Li-ion batteries andbiosensors [21] and adsorbents for dyes [22–26]. Many studieshave explored the use of agricultural wastes such as coconut shells,coffee beans, sugar cane bagasse, oil-palm stone to produce acti-vated carbons [27–38]. Coconut shells are popular as they possesshigh hardness [39], high volatility and low ash content [39–42] and

A. Jain et al. / Chemical Engineering Journal 273 (2015) 622–629 623

can yield high specific surface areas. However, synthesis of meso-porous carbon is a resource-intensive process. In particular, zincchloride (ZnCl2) and phosphoric acid (H3PO4) are extensively usedas chemical activating agents [43–47] for the production of meso-porous activated carbon from biomass, and excess quantities of theactivating agents are required for formation of high mesoporosityand surface area [23,48,49].

Chemical activation has been shown to be influenced by thepresence of oxygenated functional groups (OFGs) comprising car-boxylic, lactonic and phenolic moieties in the precursor[20,50,51]. Biomass and pure carbohydrates have been hydrother-mally carbonized to improve the chemical characteristics of hydro-char products for various applications by bestowing OFGs andreducing the degree of aromatization [20,27,32,52–54]. Liu et al.reported a 340% increase in OFG content in hydrothermally treatedpinewood compared to that observed in pinewood char obtainedby pyrolysis. Higher Cu adsorption on the former was attributedto the presence of higher OFG content, thereby demonstratingthe importance of OFGs in adsorption of cationic species [55].However, it is well-known that the hydrochars thus produced pos-sess significantly low surface areas compared to activated adsor-bents [13,14]. Thus, in the absence of further processing,hydrothermal carbonization as a stand-alone process is not suffi-cient for efficient deployment of biomass as an adsorbent. Sevillaet al. validated hydrochar as a suitable precursor for microporousactivated carbon synthesis when activated with KOH [27].

Formation of activated carbon from biomass typically demandsuse of activating agents in excess. Ahmadpour et al. obtained acti-vated carbon with a BET surface area of 2500 m2/g and a total porevolume of 1.95 cm3/g from coconut shell by employing ZnCl2 asthe activating agent (ZnCl2:shell ratio = 5:1) [46]. Recently, Maet al. reported a total pore volume of 2.727 cm3/g and a BET surfacearea of 1297 m2/g by activating glucose with activating agent (mix-ture of ZnCl2 and KCl):glucose ratio of 6:1 [56]. The use of activatingagents in high excess to create mesoporosity makes such processesuneconomical and thus alternative processes that can deliver meso-porous carbon with reduced resource usage are clearly needed.

We have reported recently that hydrothermal pre-treatment ofbiomass (coconut shell) in presence of ZnCl2 to obtain hydrocharleads to extensive dehydration and increase in OFG content com-pared to the hydrothermal treatment in the absence of it asZnCl2 eases the breakage of glycosidic linkages. High OFG contentin the hydrochar makes it more suitable for chemical activationand thus results in an increase in the mesopore area of the acti-vated carbons [57,58]. It can be expected that oxidative surfacemodification of raw lignocellulosic biomass with an oxidizingagent may result in enhanced formation of OFGs, thereby leadingto improved activation. Indeed, Zeronian et al. have shown thatcellulose oxidized to oxy-cellulose upon treating with H2O2 whichmight also lead to formation of carboxylic groups [59]. In this work,we have employed H2O2 as an oxidative agent to induce formationof OFGs on the precursor during pre-treatment of raw coconutshell which is expected to increase the affinity of the chemicalactivating agent (ZnCl2) to the precursor surface during the ensu-ing activation process. Amarasekara et al. have proposed thatZn2+ exists as hydration shells in solution and the coordinatedwater molecules act as nucleophiles [60]. These nucleophiles areexpected to be attracted to the free electron pairs of the O atomsin the OFGs, thereby giving Zn2+ greater access to the precursorsurface which leads to better dehydration of lignocellulosicbiomass. Thus, the OFGs promote chemical activation by ZnCl2,leading to higher mesopore area and volume. While ZnCl2 isdeployed as a catalyst in hydrothermal pre-treatment and also asthe activating agent during pyrolysis in this work, most of it canbe recovered and reused [36,46,61–63] and hence its use doesnot impose limitations in terms of environmental issues.

H2O2 was presented to the raw biomass by refluxing andhydrothermal treatment and the ensuing product was subjectedto activation with ZnCl2. Use of H2O2 in resulted in improvedmesopore area and in particular, its use in the hydrothermalenvironment resulted in increase in mesopore area up to 100%(1208 m2/g vs 606 m2/g in the absence of H2O2). This enhancementin mesopore area resulted in the large adsorption capacity ofRhodamine B (tracer dye) up to 714 mg/g. The relation betweenfunctional property of the starting material and surface character-istics of carbon is manifested as a strong interdependence betweenOFGs in the precursor and mesopore area in the corresponding car-bon. The study underscores the utility of simple pre-treatmentprotocols that extract even higher value from an established bio-mass source by understanding and exploiting the role of surfacefunctional groups.

2. Materials and experimental procedure

Coconut shells (Cocos nucifera) were obtained from Malaysia.ZnCl2, reagent grade (Scharlab), H2O2 (30% GR, Merck), blackstrapmolasses (Nature’s glory), Na2HPO4 (>99%, Sigma Aldrich), H3PO4

(85%, Mallinckrodt), sodium hydroxide (Merck, EMSURE, >99%),Rhodamine B (Sigma Aldrich, India) and hydrochloric acid (37%,Panreac) were used as received.

2.1. Preparation of activated carbon precursors

The coconut shells (after trimming the fibers) were dried at105 �C for 24 h, crushed using a commercial laboratory blender(Waring) and then ground and sieved into coarse granules(10–20 mesh). ZnCl2 and CO2 were used as chemical and physicalactivating agent, respectively and H2O2 was used as an oxidizingagent. Fig. 1 depicts the various treatments applied to raw biomassto obtain the precursors. Precursors were prepared via followingpre-treatments:

(1) ZnCl2 soaking at 105 �C (denoted by ZS).(2) Reflux with H2O2 (HR).(3) Hydrothermal treatment with H2O2 (HHT).(4) Hydrothermal treatment without H2O2 (control sample for

determining effect of H2O2 treatment) (HT).(5) Hydrothermal treatment with ZnCl2 (ZHT).

2.2. ZnCl2 soaking (ZS)

The samples (coconut shell granules or precursors) were mixedwith ZnCl2 solution with a ZnCl2:raw shell ratios of 1:1, 2:1 or 3:1and dried at 105 �C for 12 h.

2.3. Reflux with H2O2 (HR)

A mixture of coconut shell granules and H2O2 (15 g shell in90 mL H2O2 (10% by weight)) was refluxed for 1 h at 100 �C. Thesolids were separated by filtration and washed thoroughly withde-ionized water. The product was then dried at 105 �C for 12 h.

2.4. Hydrothermal treatment (HT)

2.4.1. Hydrothermal treatment with H2O2 (HHT)The coconut shell granules–H2O2 mixture (same quantities as

used in Section 2.3) was subjected to hydrothermal treatment ina Parr 4848 autoclave at 200 �C for 20 min. The reactor was thencooled to room temperature and the products were dried at105 �C for 12 h. 200 �C was chosen as the preferred treatment tem-perature since it delivered the hydrochar with high OFG content. A

Fig. 1. Treatment procedures to obtain precursors prior to physicochemical activation.

624 A. Jain et al. / Chemical Engineering Journal 273 (2015) 622–629

control sample of hydrochar (HT) was produced using the sameconditions but in the absence of H2O2.

2.4.2. Hydrothermal treatment with ZnCl2 (ZHT)Hydrochar obtained from hydrothermal treatment of 25 g of

raw shell was mixed with 150 mL water and ZnCl2 to obtainZnCl2:raw shell ratios of 2:1 or 3:1. The mixed samples were trea-ted at the desired temperature (200 �C and 275 �C for ZnCl2:rawshell ratios of 3:1 and 2:1, respectively) for 20 min in the Parr auto-clave. The reactor was then cooled to room temperature and theproducts were dried at 105 �C for 12 h.

2.5. Physico-chemical activation (P)

The precursors were activated by physico-chemical activationusing the procedures described previously [23,49,57,58,64]. Theprecursor was loaded on to alumina boats inside a quartz tubeplaced in a furnace (Carbolite). The temperature was ramped to800 �C at a rate of 10 �C/min in the presence of N2 at a flow rateof 50 mL/min. N2 was then replaced by CO2 at a flow rate of40 mL/min for 2 h. The furnace was cooled to room temperaturein the presence of N2 at a flow rate of 50 mL/min. The productwas stirred for 30 min in 250 mL hydrochloric acid (about0.1 mol/L), and washed with abundant distilled water until a pHof 6 was obtained for the rinse. Finally, the activated carbon wasdried at 105 �C for 24 h and used for analysis and adsorptionexperiments.

2.6. Nomenclature

The nomenclature used indicates the kind of pre-treatment,ZnCl2:raw shell ratio and the sequence of steps employed. Forexample, ZS-[2]-P was prepared by ZnCl2 soaking (ZS) of the shellwith ZnCl2 (ZnCl2:raw shell ratio = 2:1) followed by activation.HR-[3]-P refers to the sample that underwent reflux with H2O2 fol-lowed by ZnCl2 soaking (ZS) with ZnCl2:raw shell ratio of 3:1 andsubsequent activation. HHT-[2]-P refers to the sample that washydrothermally pre-treated with H2O2 followed by ZnCl2 soaking(ZS) with ZnCl2:raw shell ratio of 2:1 and subsequent activation.HHT–ZHT–[2]–P refers to the sample that underwent successivehydrothermal pre-treatments with H2O2 and ZnCl2 (ZnCl2:rawshell ratio = 2:1) and subsequently activated.

2.7. Characterization

Boehm titration was carried out for estimation of the OFGswhich are typically carboxylic, lactonic and phenolic [52,57,65].Briefly, samples were heated up at 150 �C for 24 h in presence ofa Nitrogen inert (N2) atmosphere. After cooling to room tempera-ture, 1.5 g of the sample was mixed with 50 mL NaOH (0.05 M)and the mixture was agitated by shaking for 24 h. The carbonwas removed by filtration and 10 mL aliquots were titrated with0.05 M HCl to obtain the OFG content. FT-IR spectroscopy was alsocarried out for qualitative analysis of OFGs. Nitrogen sorption iso-therms were obtained with N2 at 77 K after degassing the carbonsat 150 �C under N2 atmosphere for 12 h using a gas sorption ana-lyzer (Nova-3000 Series, Quantachrome). The surface area was cal-culated by applying the Brunauer–Emmett–Teller (BET) model tothe isotherm data points of the adsorption branch in the relativepressure range p/p0 < 0.3. The pore size distribution was calculatedfrom N2 sorption data using the nonlocal density functional theory(NLDFT) equilibrium model method for slit pores. In addition, thetotal pore volume was estimated at a relative pressure of about0.98 and the micropore surface area and micropore volume weredetermined using the t-plot method. The mesopore area was thencalculated by subtracting the micropore area from the BET surfacearea [57,58].

2.8. Adsorption studies

Molasses test was performed for the samples as previouslydescribed [57,66,67]. Briefly, the test solution was prepared by dis-solving 5 g of Blackstrap molasses in a Na2HPO4 solution (7.5 g ofNa2HPO4 in 250 mL of water). Sufficient H3PO4 was added to obtaina pH of 6.5, and the solution was diluted to 500 mL. 50 mL of thetest solution was added to 0.5 g of activated carbon and the mix-ture was stirred and brought to boil. The solution was analyzedby UV absorbance at 420 nm after filtration through a WhatmanNo. 4 filter paper. The Percentage Molasses Color Removed(PMCR) was estimated using the equation below:

PMCR

¼ ðAbsorbance of blank�Absorbance of sample with carbonÞ � 100Absorbance of blank

Table 3Oxygenated functional groups (OFGs) (meq/g) of different precursors prepared underdifferent hydrothermal temperature conditions. HHT, hydrothermally treated pre-cursor in the presence of H2O2.

Sample HHT @ 150 �C HHT @ 200 �C HHT @ 275 �C

OFG content (meq/g) 1.38 1.58 1.55

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Adsorption experiments of the dye (Rhodamine B) were carriedout using batch equilibration. A series of 250 mL Erlenmeyer flaskscontaining 0.05 g carbon sample and 50 mL dye solution(1000 ppm) was sealed and shaken at room temperature (orbitalshaker SH30), until equilibrium was reached. The dye solutionwas separated from the adsorbent by centrifugation at 7000 rpmfor 5 min using a Nuve centrifuge and analyzed by UV–visiblespectrophotometry.

Fig. 2. PMCR (Percentage Molasses Color Removed) of activated carbons preparedunder different pre-treatment conditions as a function of mesopore area and OFG.

3. Results and discussion

3.1. OFG content

Tables 1 and 2 show the extent of formation of OFGs in precur-sors pre-treated at different conditions. H2O2 pre-treated samples– whether by reflux or hydrothermal treatment – yielded higherOFG content due to higher extent of oxidation. The OFG contentalso increased with increased ZnCl2:raw shell ratio used in thehydrothermal treatment step (Table 2) due to ease of breakage ofglycosidic linkages and higher degree of hydrolysis and dehydra-tion [60]. The precursors obtained by hydrothermal pre-treatmentin the presence of H2O2 yielded the highest OFG content, thusemphasizing the combined advantage of the oxidative andhydrothermal environments. This is supported by FT-IR spec-troscopy which shows more acid groups from the OFGs for theH2O2-pre-treated samples (Fig. S1, Supporting information).Table 3 shows the effect of hydrothermal treatment temperatureon OFG content. The OFG content increases from 150 to 200 �Cand shows a small decrease for 275 �C which may be due to higherburn-off leading to decomposition of OFGs [68].

3.2. Molasses test

PMCR (Percentage Molasses Color Removed) is a strong func-tion of surface functionalities, pore size distribution and total area,and is therefore a reliable indicator of the mesoporosity of the acti-vated carbon. Higher capacity for molasses is favored by low OFGcontent. Fig. 2 shows that the PMCR values are consistent withthe mesopore areas which substantiate the role of H2O2 pre-treatment – especially in the hydrothermal environment – inenhancing mesopore development. The expected increase in meso-pore area with increasing ZnCl2:raw shell ratio [46] was furtherenhanced by H2O2-mediated hydrothermal treatment. The highercapacity for molasses when OFG content is low is illustrated uponcomparing samples HR–[2]–P (H2O2 incorporated by reflux andZnCl2:shell ratio of 2) and HT–ZHT–[3]–P (successive hydrothermaltreatments in H2O and ZnCl2 environments). The former shows

Table 1Oxygenated functional groups (OFGs) (meq/g) of different precursors.

Sample Rawshell

Without H2O2 With H2O2

Hydrothermaltreatment

Reflux Hydrothermaltreatment

OFGs 1.04 1.2 1.29 1.58

Table 2Oxygenated functional groups (OFGs) (meq/g) of precursors prepared by successivehydrothermal treatment.

ZnCl2:rawshell ratio

Successive hydrothermaltreatment without H2O2

Successive hydrothermaltreatment with H2O2

2:1 1.55 1.613:1 1.61 1.63

higher PMCR value despite having a lower mesopore area due tothe dominance of OFG effect.

3.3. BET surface area and mesoporosity

3.3.1. Advantageous effect of H2O2 mediated pre-treatment over otherpre-treatments

Fig. 3 shows the effect of OFG content in the precursors (pre-pared using different pre-treatments) upon the mesopore area ofcorresponding carbons. The mesopore area of the activated carbonincreased with increasing OFG content in the precursor. The extentof increase in mesopore area with increase in ZnCl2:raw shell ratiodepended upon the different pre-treatments of the biomass whichyielded different amounts of OFG on the precursor. Mesopore areaincreased by 58% upon increasing ZnCl2:raw shell ratio from 2:1 to3:1 when the carbons were prepared in the absence of both H2O2

and hydrothermal treatment (ZS–[2]–P vs ZS–[3]–P). Successivelylarger increases were observed when hydrothermal pre-treatmentwas carried out in the absence of H2O2 (HT–[2]–P vs HT–[3]–P),when H2O2 was introduced in the pre-treatment step by reflux(HR–[2]–P vs HR–[3]–P) and hydrothermal treatment (HHT–[2]–Pvs HHT–[3]–P)). Thus, inclusion of H2O2 in the pre-treatment pro-cess enhances the mesopore area significantly at a higherZnCl2:raw shell ratio due to the presence of more OFGs.

Fig. 4 shows the effect of pre-treatment conditions on the OFGcontent in the precursors and on the surface areas (BET and meso-pore) of the corresponding activated carbons. The strong correla-tion between the OFG content on the precursor and mesoporearea in the corresponding carbons can be observed, clearly demon-strating the importance of the former in creating the latter. Further

Fig. 3. Effect of OFG content in precursors on enhancement in mesopore area atdifferent ZnCl2:raw shell ratios.

Fig. 4. OFG content in precursors and surface areas of corresponding carbons: Effectratio = 3:1. ZS, soaking in ZnCl2 solution; HR, H2O2 reflux; HT, hydrothermal treatment;

626 A. Jain et al. / Chemical Engineering Journal 273 (2015) 622–629

details of porosity are presented in Supporting information (TablesS1 and S2).

3.3.2. Effect of successive pre-treatment with H2O2 and ZnCl2 (HHT–ZHT–P and HT–ZHT–P)

As an illustration of the importance and effectiveness of H2O2

mediated hydrothermal treatment, it can be seen that mesoporearea increased by up to 91% for the activated carbon successivelytreated with H2O2 and ZnCl2 in a hydrothermal environment rela-tive to that which underwent hydrothermal treatment with ZnCl2

alone (HHT–ZHT–[3]–P vs ZHT–[3]–P (Table 4). An increase inmesopore area of 140% is observed when hydrothermally pre-trea-ted samples are compared with those that were subjected only toZnCl2 soaking (HHT–ZHT–[2]–P vs ZS–[2]–P) (Table S1, Supportinginformation).

When the OFG content as measured by Boehm titration isrelated to mesopore area, the hydrochar precursor that underwentsuccessive hydrothermal treatments in the absence of H2O2

(Table 2) contain more OFGs than that obtained for the hydrocharprepared by single hydrothermal treatment in the presence ofH2O2 (Table 1). However, the mesopore area of the activated car-bon prepared from the former hydrochar was less than that

of pre-treatment conditions. (a) ZnCl2:raw shell ratio = 2:1; (b) ZnCl2:raw shellHHT, hydrothermal treatment in H2O2.

Table 4Textural properties and yield of carbon derived from activation of hydrothermally pre-treated precursors in the presence and absence of H2O2 followed by successivehydrothermal pre-treatment with ZnCl2.

Sample BET Surface area Mesopore surface area Total pore volume Mesopore volume Yield (%) Areame/Areat (%)

ZHT–[2]–P 1652 ± 132 640 ± 53 1.29 ± 0.01 0.768 ± 0.094 22 ± 2 38HT–ZHT–[2]–P 1775 ± 112 376 ± 24 1.11 ± 0.06 0.44 ± 0.02 28 ± 2 21HHT–ZHT–[2]–P 1947 ± 156 916 ± 40 1.71 ± 0.1 1.15 ± 0.07 18 ± 1 47ZHT–[3]–P 1666 ± 61 664 ± 27 1.33 ± 0.00 0.79 ± 0.03 33 ± 2 40HT–ZHT–[3]–P 2050 ± 252 658 ± 87 1.5 ± 0.2 0.8 ± 0.12 25 ± 3 32HHT–ZHT–[3]–P 1915 ± 145 1267 ± 133 2.1 ± 0.27 1.69 ± 0.25 18 ± 2 66

0 20 40 60 80 1000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

dV(d

)(cc

/Å/g

)

Size(Å)

ZS-[2]-P HR-[2]-P HHT-[2]-P HHT-ZHT-[2]-P

Fig. 5. Pore size distribution based on NLDFT model: carbons prepared in thepresence and absence of H2O2 pre-treatment (reflux and hydrothermal conditions).

Table 5Adsorption capacity for Rhodamine B (initial concentration used is 1000 ppm) andOFG content of activated carbons.

Activated carbon Rhodamine B adsorbed (mg/g) OFG content (meq/g)

HR–[2]–P 410 0.31HHT–[2]–P 515 0.44HT–ZHT–[2]–P 397 0.54HHT–ZHT–[2]–P 714 0.46

A. Jain et al. / Chemical Engineering Journal 273 (2015) 622–629 627

obtained from the latter (HHT–[3]–P > HT–ZHT–[3]–P, Table 4).The anomalous observation of reduced mesoporosity for the sam-ple that underwent two successive hydrothermal treatments maybe due to the reduced the solubility of ZnCl2 caused by acidwash-out after first hydrothermal treatment.

On the other hand, the hydrochar precursor that underwentsuccessive hydrothermal treatments in the presence of H2O2

(HHT–ZHT) resulted in higher mesopore area than that obtainedfrom the hydrochar prepared by single hydrothermal treatment(HHT). This is because the increase in the OFG content in thehydrochar due to the incorporation of H2O2 compensates for thereduced solubility of ZnCl2 due to acid wash-out. Funke et al. havealso reported the release of acids during hydrothermal treatment[69] and Prauchner et al. have shown that solubility of ZnCl2 isenhanced under acidic conditions [70]. In particular, the use ofhydrothermal pretreatment in the presence of H2O2 yielded highermesopore areas even with lower ZnCl2:raw shell ratios. This is evi-dent from the larger mesopore area obtained for carbon preparedwith successive hydrothermal treatments for a ZnCl2:raw shellratio of 2:1 compared to the sample formed by a similar processin the absence of H2O2 and with a ZnCl2:raw shell ratio of 3:1(HHT–ZHT–[2]–P > HT–ZHT–[3]–P).

3.4. Pore size distribution

Fig. 5 shows the effect of reflux, hydrothermal treatment andsuccessive hydrothermal treatment steps on the pore size dis-tribution when a ZnCl2:shell ratio of 2:1 was employed.Enhanced porosity and wider pore size distribution were obtainedby H2O2 treatment by reflux; the porosity in the range of 20–50 Åincreased further when the sample was hydrothermally pre-treated with H2O2, and even further with wider distribution when

successive hydrothermal treatments with H2O2 and ZnCl2 was car-ried out (HHT–ZHT–[2]–P).

3.5. Adsorption of Rhodamine B on mesoporous carbons

Results of batch equilibrium studies (Table 5) shows adsorptioncapacities of various carbons obtained using hydrochars preparedunder different conditions. Carbon prepared by using successivehydrothermal treatment in the presence of H2O2 (HHT–ZHT–[2]–P) resulted in the highest adsorption capacity for Rhodamine B.The trend for adsorption by the carbons was in accordance to theirrespective mesopore surface areas i.e. HHT–[2]–P > HR–[2]–P > HT–ZHT–[2]–P. Adsorption of dyes is also dependent on thesurface functionalities viz., OFG content. High OFG content is favor-able for higher Rhodamine B (cationic dye) uptake. Although HT–ZHT–[2]–P has the highest OFG content, which would normallybe expected to yield the highest adsorption capacity, the observedcapacity for Rhodamine B is the lowest, which is a clear indicationof the importance of mesoporosity which dominated over OFGcontent of activated carbons. Relative to the highest capacityobtained in this work (714 mg/g) from a Rhodamine B concentra-tion of 1000 ppm, Guo et al. have obtained adsorption capacity of0.95 mmol/g (�455 mg/g) for Rhodamine B on rice husk based car-bon using the same initial dye concentration of 1000 ppm [71]. Ourcapacity is comparable with that delivered by ordered mesoporouscarbons (uptake capacity of 785 mg/g) obtained by the more oner-ous and expensive templating process [22].

4. Conclusions

H2O2-mediated hydrothermal pre-treatment of biomass(coconut shells) resulted in formation of high-surface area meso-porous activated carbons. The advantage in using H2O2 to increasethe oxygenated functional group (OFG) content in precursor is evi-dent from the increase in mesopore area of 40% observed whenH2O2 was introduced under reflux conditions. An even higher ofincrease up to 100% is obtained when H2O2 is presented to the bio-mass in a hydrothermal environment. The high mesopore surfaceareas are reflected in the observed high adsorption capacity forRhodamine B up to 714 mg/g. A strong interdependence isobserved between the OFG content in the precursors and mesoporearea in corresponding carbons at constant ZnCl2 content. UsingH2O2 (in general, and particularly, in a hydrothermal environment)creates more OFGs that increase the efficiency and effectiveness of

628 A. Jain et al. / Chemical Engineering Journal 273 (2015) 622–629

chemical activation in the production of high surface area meso-porous activated carbon from biomass at reduced cost.

Acknowledgements

The financial support received from NUS, Singapore andNational Environment Agency (Environmental TechnologyResearch Programme) of Singapore for the pursuit of this projectis gratefully acknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2015.03.111.

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